High-resolution detectors used in space astronomy and in some laboratory and industrial applications must be cooled to extremely low temperatures (<0.1 K) in order to obtain the required sensitivity. For space applications, adiabatic demagnetization refrigerators (ADRs) have an inherent advantage over other techniques because they don't rely on gravity to function, and because they have higher thermodynamic efficiency. An ADR stage produces cooling (or heating) by the interaction of a magnetic field with the magnetic spins in a paramagnetic salt. Magnetizing the salt produces heating, and demagnetizing the salt produces cooling.
A conventional “single-shot” ADR consists of a “salt pill” containing the magnetic salt, a superconducting magnet, and a heat switch. The salt pill is located in the bore of the magnet, and the heat switch links it to a heat sink. Regardless of the initial conditions, the refrigeration cycle consists of the following steps. First, the salt pill is magnetized, causing it to warm up. Second, when its temperature exceeds that of the heat sink, the heat switch is powered into the on state. Third, the salt continues to be magnetized, generating heat which flows to the sink. This continues until full field is reached, which necessarily is strong enough to significantly align the spins and suppress the entropy of the salt. Fourth, at full magnetic field, the heat switch is deactivated to thermally isolate the salt from the heat sink. Fifth, the salt is demagnetized to cool it to the desired operating temperature. In general, the salt will then be receiving heat from components parts. The heat is absorbed and operating temperature maintained by slowly demagnetizing the salt at just the right rate. Heat can continue to be absorbed until the magnetic field is reduced to zero, at which point the ADR has run out of cooling capacity.
Conventional single-shot ADRs provide cooling for a period of time, then they must be taken off line and recycled. The periodic interruption in cooling leads to a number of significant constraints on the design of ADRs for space missions. The two most important are that the hold time must be long, typically on the order of one day, and the time spent recycling the ADR must be short, typically one hour or less.
A need exists for an ADR salt pill design and a method for making the same that overcome these constraints.